When two surfaces are brought in contact with each other, a transfer of electrons may occur resulting in a residual static electrical charge when the surfaces are separated. This phenomena is known as triboelectricity. If the surface is composed of a material that is a conductor, the electrons will dissipate quickly thereby eliminating the excess charge. On the other hand, if the surface is composed of a material that is an insulator (a dielectric), the surface charge takes much longer to dissipate.
Thermoplastic polymers, however, are typically excellent insulators, having an extremely high surface resistivity of more than 10.sup.14 ohms/square, and thus they are unsatisfactory for uses that require an antistatic nature. As the polymers are nonconductive, they accumulate high charges promoting an attraction for dust and dirt, and they can discharge to any lower potential body with which they come in contact. To modify a polymer so that it will have antistatic characteristics, the resistivity of it must be decreased, i.e. The conductivity is increased which in turn causes an increase in the rate of static dissipation. Increase in conductivity has been accomplished in the past by the use of antistatic agents to promote static-charge decay of surfaces thereby reducing clinging effect, eliminating spark discharge, and preventing accumulation of dust.
It is well known that static charge can be reduced by increasing the moisture content of the atmosphere, and thus the approach in the past has been to use an antistatic agent which will chemically modify the polymer to impart hydrophillic properties to it by providing functional groups that attract moisture to it. For instance, it is well known to apply external antistatic agents onto polymers by conventional coating methods. Also, it is well known to apply internal antistatic agents which are volume dispersed in the polymer; i.e. incorporated into the polymer by compounding or extrusion prior to or during molding or film-forming operations. These agents work by migrating to the polymer surface. This migration is colloquially referred to in the art of polymer chemistry as a "blooming" effect. When the antistatic agent has not remained volume dispersed but instead has bloomed to the surface, the mechanism for moisture attraction is the same as with the external antistatic agents. The atmospheric moisture is attracted causing decay of static charges. Accordingly a high rate of blooming is required.
The following known antistatic agents appear to function in the above-mentioned manner.
An example of an external antistatic agent is described in U.S. Pat. No. 3,223,545 to Gallaugher et al which discloses a dialkanol amide of the formula ##STR1## wherein
R is a C.sub.6 to C.sub.16 alkyl and
n is an integer from 2 to 4, dispersed in a volatile liquid which is applied to the surface of a solid polymer.
One example of an internal antistatic agent is described in U.S. Pat. No. 3,220,985 to Breslow which discloses modifying hydrocarbon polymers with a monosulfonazide of the formula RSO.sub.2 N.sub.3, where R is an organic radical inert to the modification reaction. For instance, to an acetone slurry of finely divided polypropylene is added para-toluene sulfonazide, followed by agitation at room temperature to evaporate the acetone solvent. The resultant is then heated at 160.degree. C. for 2 hours.
Another internal antistatic agent is described in U.S. Pat. No. 3,164,481 to Shibe which discloses combining a quaternary ammonium benzosulfimide with a plastic. (For clarity, it is mentioned benzosulfimide is also known as saccharin.) For instance, in Shibe is disclosed Epolene E (a polyethylene supplied by Eastman Chemical Products, Inc., Kingsport, Tenn.) melted together with dodecyl benzyl trimethyl ammonium benzosulfimide and the molten resultant is spread out in a sheet.
Also of interest is the internal antistatic agent described in U.S. Pat. No. 3,441,552 to Rombusch et al. The patent discloses incorporating an alkoxypropylamine of the formula ##STR2## into a polyolefin where R.sub.1 represents an alkyl, alkenyl, alkylcycloalkyl, aryl, alkylaryl, or alkenylaryl group of 6 to 25, preferably 8 to 18 carbon atoms in the alkyl or alkenyl moieties and 4 and 18, preferably 6 to 12 carbon atoms in the cycloalkyl moiety, and 6 to 14, preferably 6 to 10 carbon atoms in the aryl moiety; R.sub.2 and R.sub.3 can each represent a hydrogen atom, or an alkyl or alkenyl group of 1 to 5 carbon atoms. For instance, 100 g of octadecyloxy-propyl-N,N-dimethylamine are homogenized in a blender with 10 kg of polypropylene. The resultant is granulated and injection mold plates are produced from the granulation.
The following patents are also of general interest.
For instance, U.S. Pat. No. 4,536,532 (1985) to Miller, relates to a process for the manufacture of a polyvinyl alcohol homopolymer having a vinyl alcohol content in excess of 95% wherein said homopolymer is mixed with a plasticizer selected from the group consisting of N-substituted C.sub.10-20 fatty acid amides; aryl, alkaryl, N-aryl aryl, N-alkaryl aryl and N-alkyl alkaryl sulfonamides and alkaryl sulfonamides; N-alkyl pyrrolidones; sulfonated alkyl phenols; aryl and alkaryl phosphates and phosphites; alkylene carbonates and selected mixtures thereof.
Also of general interest is the blend of high melting nylon (melting point between 415.degree. and 440.degree. F.) and ethylene vinyl alcohol copolymer (EVOH) plasticized with lauramide, o,p-toluenesulfonamide, N-ethyl-o,p-toluenesulfonamide or a polyamide of 7000-10000 molecular weight, as described in U.S. Pat. No. 4,347,322 to Odorzynski et al.
Also of general interest is U.S. Pat. No. 4,605,684 (1986) to Pcolinsky which relates to a method of preparing a flexible polyurethane foam from a polyol and a polyisocyanate the improvement which comprises adding to the foam-forming composition from about five to about 25 parts by weight per 100 parts by weight of polyol of an antistatic additive composition comprising one part by weight of a quaternary ammonium compound selected from the group consisting of soya dimethyl ethyl ammonium ethylsulfate, soya dimethyl ethyl ammonium ethylphosphate, and mixtures thereof and from about 0.4 to about 3 parts by weight of a plasticizer composition selected from the group consisting of N-ethyl-o- and p-toluene sulfonamide, o- and p-toluene sulfonamide, tetrakis (2 chloroethyl) ethylene diphosphate, and mixtures thereof, to provide a foam having a reduced tendency to develop and accumulate electrostatic charges.
The antistatic agents useful in the present invention are substantially non-hygroscopic and substantially non-migratable. Thus, unlike with the previous antistatic polymeric compositions, the antistatic properties of the antistatic polymeric composition of the present invention are substantially independent of ambient relative humidity and the agent substantially remains volume dispersed in the polymer. Accordingly, the problem of surface contamination and corrosion when a static sensitive device was in contact with the previous antistatic polymeric compositions is obviated. For instance, the present polymer containing the agent may be made into a single and/or multiply film. Such films have an increased tendency to dissipate electrostatic charges. For instance, the composition may be extruded together with polymers known for their strength such as ethylene vinyl acetate (EVA) or linear low density polyethylene (LLDPE), which film is useful for making packaging, such as a bag or over-wrap, for electronic devices that are sensitive to static electricity. Such a film may also be fashioned to have cushioning characteristics by using bubble cap (also known as air cushioning) machinery such as that described in U.S. Pat. No. 4,576,669 and U.S. Pat. No. 4,579,516, both to Caputo, to make a bubble cap package which provides cushioning and then bagging or wrapping a circuit board therewith. Such a cushioning bubble cap material is also useful in lining a portable work station used for storage and transportation of static electricity sensitive devices such as the work stations disclosed in U.S. Pat. No. 4,494,651 issued in 1985 to Malcolm. Also such a film, which may or may not be in a cushion form, is useful to make a package for devices in a medical operating room where explosive oxygen and/or ether are present and thus protection from static electricity must be provided.
Therefore, it is an object of the present invention to provide an antistatic polymeric composition wherein the antistatic agent is substantially non-migratable and substantially remains volume dispersed in the polymer. It is a further object to provide the antistatic polymeric composition in single-ply or multi-ply film useful for wrapping static sensitive devices. It is also an object to provide such films having an increased tendency to dissipate electrostatic charges.